U.S. Pat. Nos. 4,640,352 and 4,886,118 disclose conductive heating of subterranean formations of low permeability that contain oil to recover oil therefrom. Low permeability formations include diatomites, lipid coals, and oil shales. Formations of low permeability are not amiable to secondary oil recovery methods such as steam, carbon dioxide, or fire flooding. Flooding materials tend to penetrate formations that have low permeabilities preferentially through fractures. The injected materials bypass most of the formation hydrocarbons. In contrast, conductive heating does not require fluid transport into the formation. Oil within the formation is therefore not bypassed as in a flooding process. When the temperature of a formation is increased by conductive heating, vertical temperature profiles will tend to be relatively uniform because formations generally have relatively uniform thermal conductivities and specific heats. Transportation of hydrocarbons in a thermal conduction process is by pressure drive, vaporization, and thermal expansion of oil and water trapped within the pores of the formation rock. Hydrocarbons migrate through small fractures created by the expansion and vaporization of the oil and water.
U.S. Pat. No. 5,255,742 discloses a flameless combustor useful for heating subterranean formations that utilizes preheated fuel gas and/or combustion air wherein the fuel gas is combined with the combustion air in increments that are sufficiently small that flames are avoided. Creation of NO.sub.x is almost eliminated, and cost of the heaters can be significantly reduced because of less expensive materials of construction. Preheating the fuel gas according to the invention of patent '742 results in coke formation unless C0.sub.2, H.sub.2, or steam is added to the fuel gas. Further, start-up of the heater of patent '742 is a time consuming process because it must operate at temperatures above the uncatalyzed autoignition temperature of the fuel gas mixture.
Catalytic combustors are also known. For example, U.S. Pat. No. 3,928,961 discloses a catalytically-supported thermal combustion apparatus wherein formation of NO.sub.x is eliminated by combustion at temperatures above auto-ignition temperatures of the fuel, but less than those temperatures at which result in substantial formation of oxides of nitrogen.
Metal surfaces coated with oxidation catalyst are disclosed in, for example, U.S. Pat. Nos. 5,355,668 and 4,065,917. These patents suggest catalytic coated surfaces on components of a gas turbine engine. Patent '917 suggests use of catalytic coated surfaces for start-up of the turbine, and suggests a mass transfer control limited phase of the start-up operation.
It is therefore an object of the present invention to provide a combustion method and apparatus which is flameless, and does not require additives in a fuel gas stream to prevent formation of coke. In another aspect of the present invention, it is an object to provide a combustion method and apparatus wherein formation of NO.sub.x is minimal. It is also an object of the present invention to provide a flameless combustor wherein fuel and oxidant can be combined initially, and distribution of combustion determined by distribution of catalytic surfaces within a combustion chamber.